Refrigeration devices are a kind of apparatus that cools objects or maintains them at a reduced temperature. Cooling and refrigeration systems take many forms depending on the application for which they are used. Conventional household refrigerators designed to temporarily preserve foods operate around the freezing temperature of water, 273 degrees Kelvin (273K), and operate on a closed-cycle that evaporates and condenses a refrigerant. Scientific, medical and industrial refrigeration systems may rely on cooling provided by liquified gases. Liquid nitrogen based cooling devices generally provide temperatures around the transition temperature of this substance, 77K. Liquid helium based cooling systems generally provide temperatures around the transition temperature of this substance, 4K, and are used to maintain superconductivity in medical imaging systems as one example of their use.
However, some applications require even lower temperatures than those afforded by liquid gas based cooling systems or conventional refrigerators. Examples are scientific systems where even a slight elevation in temperature could adversely impair the operations of a scientific instrument such as a photon detector or a sensitive telescope. Such systems are described in the prior art, for example Devlin, et al., in Cryogenics 44 (2004) and demonstrate capabilities to reach within several hundred milli-Kelvins of absolute zero (0K). In order to reach such low temperatures, cryogenic systems usually employ multiple stages, each providing cooling to a successively lower temperature than its preceding stage and may culminate in one or more helium adsorption pump-evaporator units.
Cryogenic refrigeration systems have been in development and use since the 1970s for cooling materials, experiments and instruments to very low temperatures in order to take advantage of material properties at such temperatures. Many technologies have emerged to generate increasingly lower temperatures, from 100K all the way down to 2-10 mK (as in the case of helium dilution refrigerators) or even lower (through nuclear demagnetization cooling). The more extreme their capabilities, however, the larger and costlier cryogenic refrigeration becomes, often requiring the consumption of expensive cryogens such as liquid helium. In order to reduce the size and cost of cryogenic cooling while maintaining significant cooling performance, cryogenic adsorption refrigerators (sometimes referred to as “sorption fridges”) have been developed which contain no moving parts, self-contain their necessary refrigerant(s) and can be constructed in very compact shapes.
The principle of operation of sorption refrigeration device derives from their ability to exploit adherence and absorption (collectively causing adsorption) in some substances under some conditions. The result is a component in the adsorption refrigerator that causes a thin film of a gas or liquid solute to adsorb onto a surface of a solid sorbant material. A typical adsorption refrigerator has several main components. These include a sorbent filled pump, a condenser, and an evaporator, usually connected to one another by a gas-filled tube, and these components are kept under elevated static pressures (e.g., several hundred psi or more). The gas can be He-3 or He-4. The gas is cooled to its condensation temperature so that it is liquified in the evaporator portion of the system. Meanwhile, the gas is adsorbed in the pump portion of the refrigerator during operation (e.g., using a thermal switch) so as to lower the pressure in the evaporator portion of the refrigerator. This causes the liquified gas (e.g., liquid helium) to evaporate from the evaporator portion of the system. When all of the liquified gas evaporates to return to its gaseous state, the pump is heated, and the gas is desorbed. The cycle is repeated to continue the operation of the refrigerator.
Sorption refrigerators are (in their simplest form) based on an on-time/off-time duty cycle: during the off-time, gaseous refrigerant is extracted from the adsorber and condensed within the evaporator (or “pot”), until there is significant refrigerant condensed within the pot. When the adsorber is switched from a desorbing mode to an adsorbing mode, the resulting negative pressure pumps on the refrigerant in the pot and causes it to evaporate, taking heat from the cold head surface of the pot, cooling it below the temperature of the liquid refrigerant.
The materials and construction of the adsorber, and the method of controlling the adsorbing and desorbing behavior of the adsorber determine the operation of a sorption refrigerator. Activated carbon or charcoal are often used as the adsorber material as they are affordable, have high surface area for adsorbing gasses and their adsorb/desorb behavior is easily controlled by manipulating the temperature of the carbon or charcoal. This manipulation, in the case of many cryogenic sorption fridges, is accomplished with a paired heater and thermal switch. The heater is responsible for bringing the adsorber to its desorbing temperature, while the thermal switch is responsible for thermally connecting the adsorber to a cooling source (e.g. a liquid helium bath or cryorefrigerator) in order to cool the adsorber to an adsorbing temperature. The adsorber can adsorb and desorb gas up to the carrying capacity of the adsorber itself, providing the pot of the sorption fridge with a set quantity of refrigerant that can be evaporated to provide cooling.
Gaseous refrigerant, however, will not spontaneously become liquid, thus there is a need for a condenser unit that accepts the flow of gas from the adsorber, and upon contact with the gas molecules, provides significant cooling such that the gas condenses to its surface and flows towards the evaporator pot. This is achieved typically by thermally coupling a discrete portion of the sorption fridge between the adsorber and evaporator to a cooling source (e.g. a liquid helium bath or cryorefrigerator). When the adsorber is located above the pot, the condensed liquid refrigerant flows in the direction of gravity towards the pot and collects within.
In the general case, the evaporator pot of a sorption fridge must serve two purposes. Firstly, it must hold a sufficient quantity of condensed refrigerant such that the amount desorbed from the adsorber does not overfill the pot; should the pot overfill, evaporation of refrigerant will occur outside the pot itself and will be too far away from the cold stage of the pot to provide cooling to that desired area. Secondly, the pot should have a means of controlling the evaporation rate such that there is a stabilizing effect on the temperature of the cold surface, and that there is a compromise between the achieved low temperature and the achieved duration of the ON (adsorbing) mode.
In the conventional helium-4 sorption fridge, certain construction techniques are required to hold a sufficient amount of helium within the fridge and allow a reasonably lengthy ON duration, reasonably short OFF duration, and switching response time for a given size of sorption fridge.
FIG. 1 illustrates a prior art adsorption refrigerator 10. The refrigerator comprises an adsorbing pump chamber 100 having a pressure-capable canister or housing 102 constructed of thermally insulating material such as stainless steel except the portion to which the adsorbent material 104 (e.g. activated charcoal) is bonded. The walls of such systems have a thickness to withstand the pressures applied within the system. In single-chamber pump designs like this the walls must be relatively thick and large, causing them to be slow to react to temperature changes, having a relatively large thermal inertia. This portion is comprised of conductive metal, with heat conducting copper fins to aid heat transfer to and from the adsorbing material. The evaporator pot 120 also comprises of a pressure-capable canister 122 constructed of thermally insulating material such as stainless steel, except the cold stage 124 which is comprised of conductive metal, and also with fins to aid heat transfer to and from the liquid, evaporating refrigerant. The connecting tube 130 comprises a thin-walled tube of thermally insulating metal, such that heat can be transferred through the wall to the condenser 110 but minimizes conduction up or down the wall to and from the adsorber 100 and evaporator pot 120. The length of the connecting tube 130 must allow sufficient thermal insulation between the adsorber 100, condenser and evaporator pot 120. As a result of this conventional construction format, sorption fridges are not especially space efficient when attempting to package them into a helium dewar dipstick, or with a closed-cycle cryorefrigerator.
FIG. 2 illustrates an exemplary multi-stage cryogenic refrigeration system 20 according to the prior art. The system is housed in a housing 200. Within the housing 200 are a cryostat 210, which may include more than one stage, such as a first stage 210a and a second stage 210b. These are coupled to an adsorption refrigerator 220 like the one described above. Some unfavorable packaging and manufacturing challenges arise with such systems.
In particular, existing refrigeration systems generally require a bulky cryostat and housing along with associated mechanical support and auxiliary systems. These can be unwieldy, expensive, heavy and take up too much volume for some applications, typically requiring an outer housing diameter of 10 to 14 inches or more. Also, such systems generally offset the cryostat and the sorption refrigerator so that they are mechanically unstable and require special attachments and enclosures. With helium dewars, the sorption fridge is difficult to package in a way such that the liquid helium bath cools the condenser while leaving the adsorber thermally isolated. With cryocooler applications, packaging the sorption fridge concentrically to the cryocooler axis would result in a very long vacuum chamber, whereas packaging the sorption fridge side-by-side with the cryocooler results in a very wide or large-diameter chamber, neither of which is particularly easy or economical to manufacture.
As would be appreciated, existing systems are in constant need of refinement and innovation so as to provide lower temperatures more efficiently, more reliably and more economically in a packaging that is compact and adapted for high-performance applications. The present disclosure is direct to such innovations.